Since a nitride semiconductor has a characteristic in high saturation electron velocity, wide band gap and so forth, application of the nitride semiconductor to a semiconductor device of a high withstand voltage and high output power is examined.
For example, the band gap of GaN that is a nitride semiconductor is approximately 3.4 eV, and is greater than the band gap of Si (approximately 1.1 eV) and the band gap of GaAs (approximately 1.4 eV) and has a high breakdown field strength. Therefore, GaN is a very prospective material as a material for a semiconductor device for achieving high voltage action and high output power.
As a semiconductor device for which a nitride semiconductor is used, a field-effect transistor, particularly, a high electron mobility transistor (HEMI), is available.
For example, as a GaN-based HEMT (GaN-HEMT) , an AlGaN/GaN-HEMT for which GaN and AlGaN are used as an electron transit layer (channel layer) and an electron supply layer, respectively, is available. In the AlGaN/GaN-HEMT, a distortion arising from a lattice constant difference between GaN and AlGaN occurs with AlGaN, and high-concentration two-dimensional electron gas (2DEG) is obtained by piezoelectric polarization and a spontaneous polarization difference of AlGaN caused by the distortion. Therefore, a high-withstand-voltage and high-output power device can be implemented by the AlGaN/GaN-HEMT.